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  <title>Document</title>
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<script>
  let str = `字符串信息`;
  let num = 8;
  let arrary = [];
  let obj = {};
  let fn = function () {};
  /* 
    typeof 
      字符串返回 string
      数字返回 number
      数组，对象，函数都返回Object
      注意:声明的变量未赋值或未声明变量都时undefined
    
    instanceof
      判断原型链是否有这个属性
  */
</script>

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